The silicoflagellates, ebridians and archaeomonads are three groups of quite dissimilar organisms. However, they are sometimes discussed collectively because each group has a relatively small siliceous skeleton, and is relatively less common, and much less understood, than the diatoms or radiolarians. The silicoflagellates are the most abundant of the three groups. They usually make up one or two percent of the siliceous component of marine sediments (Riedel, 1959), and can be viewed as the least abundant of the major plankton groups. The ebridians and archaeomonads, when found at all, are generally only a minor constituent of the sediment.